Striped Barnacle
(Balanus amphitrite)

Description: The hard exterior is relatively smooth, with reddish or maroon vertical stripes. The base can grow to 20 mm wide (about the size of a nickel), and the barnacle grows about 20 mm high. While there are native species of barnacle with similar stripes, they are usually restricted to deeper water or exposed coasts.

Habitat: These barnacles can be found attached to rocks, wood, or other hard substrates (such as boat hulls) in the warmer portions of bays and harbors.

Origin: Southwestern Pacific and Indian Oceans.

Invaded Areas: San Francisco Bay, Southern California, in bays and harbors in warm seas worldwide.

Concerns: Striped acorn barnacles have successfully invaded harbors in many regions of the world, and have become problematically abundant in some of them. They are readily transported on ship hulls, and indeed can cause serious biofouling problems: studies have shown they can reduce a ship’s speed by up to 40%. They breed year-round, and thus larvae have the potential to settle on ships in infested harbors at any time. In bays and harbors, the structure created by dense aggregations of barnacles can serve as a refuge for other alien species, such as the Harris mud crab.